Carbonate reservoirs are difficult to exploit because of their heterogeneous nature. A major challenge in carbonate environments is therefore to map these heterogeneities which have a strong impact on oil and gas production. In many carbonate reservoirs, the porosity of the rock (i.e. matrix porosity) is high enough to contain large amounts of oil in place, but the permeability is mainly provided by fracture corridors, not by the intrinsic nature of the rock matrix. In other reservoirs, the oil in place is found primarily in caves and conduits formed in the rock formation by infiltration and action of rain water (so-called karst formations).
Therefore, the ability to detect these heterogeneities and possibly characterize their properties, i.e. obtaining three dimensional maps of their geometry and characteristics, is essential in these environments.
To obtain images of the subsurface, a seismic method is often used, which consists in creating and sending seismic waves in the ground using sources such as explosives or vibrator trucks on land, or airguns offshore. The seismic waves penetrate the ground and get bounced, or reflected off major geological discontinuities in the subsurface. As a result, they come back to the surface, where they are recorded using arrays of three component geophones (on land), or hydrophones (offshore) which are regularly distributed to cover areas of several square kilometres.
Seismic reflections assume that local planes are large compared to the seismic wavefront. When the subsurface contains edges and short-scale heterogeneities, the wavefront undergoes diffractions rather than reflections.
Diffraction effects are typically present with carbonate reservoirs, because of the characteristics mentioned above, i.e. the presence of faults, fissures, conduits, caves etc.
The importance of diffracted waves for obtaining better images of subsurface carbonate-type reservoirs has long been recognized.
Typically, diffracted energy is one or even two orders of magnitude weaker than the reflected one and it is not easy to distinguish diffracted events in a seismic dataset or a diffraction image in a seismic image. Therefore, diffracted and reflected energy have to be separated properly.
A suitable domain for performing this separation is the post-migration dip angle domain as disclosed by Landa et al. “Separation, imaging, and velocity analysis of seismic diffractions using migrated dip angle gathers”, SEG Expanded Abstracts, Vol. 27, pages 2176-2180, 2008. In this document, reflection and diffraction events are separated in the dip angle domain using a plane-wave-destruction method, described by Fomel: “Applications of plane-wave destruction filters”, Geophysics, 67, 1946-1960, 2002, requiring accurate estimation of the velocity model used for the migration.
Pre-stack post-migration common image gathers in the dip angle domain are generated from seismic data conventionally measured and recorded. A dip angle common image gather (DA-CIG) is a bi-dimensional data structure with a first axis representing the dip angle and a second axis representing the depth.
A DA-CIG is typically obtained for one horizontal position (x, y) by summing contributions from a number of seismic traces recorded by seismic detectors around the horizontal position (x, y). Those contributions for a depth z and a dip angle α are determined by assuming that some structure of the subsurface at position (x, y, z) has a dip angle α and bounces back seismic waves from the source. Snell's law and a model for estimating the velocity of seismic waves in the migration process determine detector positions and respective reading times for those detectors, providing contributions to the DA-CIG at (x, y) for (z, α).
If the structure at position (x, y, z) is a reflector with a dip angle α, then seismic energy is specularly reflected and yields a concave feature in the DA-CIG at (x, y) which is approximately of a parabolic shape with an apex located at (z, α).
If a diffractor rather than a reflector is the structure located at (x, y, z), energy is scattered in all directions from such structure, which results in a flat feature in the DA-CIG at (x, y) for the depth value z. Such flat feature is horizontal if the velocity model used for migration is an accurate estimation of the seismic velocities in the subsurface, and if the DA-CIG is located directly above the diffractor. It is slanted and quasi-linear if there is some error in the velocity model.
A DA-CIG 12 is illustrated in FIG. 1. The illustrated DA-CIG 12 is located above two diffraction points and is computed from measured seismic data using a correct velocity model. When viewing a DA-CIG 12, two kinds of features can thus be distinguished. The first kind consists in concave features and the second kind consists in flat features. The concave features are related to reflection events and the flat features are related to diffraction events. For instance, two horizontal features 14, 16 appear in the common image gather of FIG. 1. Both horizontal features are related to diffraction points in the subsurface.
The summation of DA-CIGs obtained for different horizontal positions produces a seismic image of the subsurface. An image of a reflector is formed by a constructive summation of these DA-CIGs in a vicinity of the apexes of the concave features, in the form of smiles, related to such reflector.
When summing the DA-CIGs to assemble the migrated seismic image, the tails of the smile-shaped reflection features are either annihilated in the migrated image due to destructive optimal summation, or they create migration noise due to aliasing in the data and truncation of the migration aperture. The ability to control migration aperture is important in order to reduce this noise in migrated images.
In “Limited aperture migration in the Angle domain”, 71st EAGE Conference, Expanded Abstracts, Amsterdam, Netherlands, 8-11 Jun. 2009, N. Bienati, et al., proposed a method using the property that specular energy of the reflection events is concentrated in a vicinity of the apex position. Their method includes detecting apices on DA-CIGs by automated picking, and muting the rest of the energy prior to summation. The migrated images obtained after such processing tend to be over-smoothed and they may thus miss small-scale structural features such as fault planes and steps. The diffraction components are removed.
In “Separation and imaging of seismic diffractions in dip-angle domain”, 72nd EAGE Conference, Expanded Abstracts, 14-17 Jun. 2010, A. Klokov A., et al., introduced the use of a hybrid Radon transform to separate diffraction and reflection features in CA-CIGs. In a first step, the apex regions of the parabolas related to the reflection events are detected and muted; then, the hybrid Radon transform is applied to separate the remaining tails parts of the parabolas from the linear-shaped diffraction features. An image of the diffractors only can thus be obtained after summation of the processed DA-CIGs.
There is a need for a method of analyzing seismic data which produces seismic images with a low migration noise and a good imaging of small-scale structures in the underground.